This invention relates to a process for the conversion of more highly functional silane impurities in organosilanes to organosiloxanes to facilitate isolation and removal of these impurities from desired product organosilanes.
High-purity organosilane monomers, particularly diorganodihalosilanes, are needed for the ever-increasing quality needs for the preparation of linear organopolysiloxanes utilized in the manufacture of silicone fluids and elastomers. As an example, the preparation of high-quality, high-performance silicone elastomers require that the linear organopolysiloxanes contain a minimum level of trifunctional and tetrafunctional branching units, preferably to levels of 100 part per million (on a weight basis) or less.
For the purposes of the instant invention, the following definitions may prove helpful in understanding. "Functionality" is a definition for the ability of siloxane species to form an end-blocking structure ("monofunctionality"), a linear structure ("difunctionality"), a branched structure ("trifunctionality"), or a network structure ("tetrafunctionality"). ##STR1## "More highly functional silane" means that a tetrafunctional silane is more highly functional than a trifunctional silane, that a trifunctional silane is more highly functional than a difunctional silane, and so on. In the preparation of high-performance silicone elastomers, as an example, difunctional siloxane monomers with a minimum of more highly functional trifunctional and tetrafunctional branching units are a necessity. A mixture of dimethyldichlorosilane and a minor amount of methyltrichlorosilane is an example of a difunctional material that contains a trifunctional impurity.
In the manufacture of organosilanes, separation of the individual species from the reaction mixture is conventionally effected by distillation. In many instances the boiling points of individual organosilane components are very close, creating a very difficult distillation operation. One of the more difficult distillation separations is a combination of a diorganodihalosilane and an organotrihalosilane. Examples of such combinations are the following mixtures:
dimethydichlorosilane/methyltrichlorosilane, PA0 methylvinyldichlorosilane/vinyltrichlorosilane, PA0 phenylmethyldichlorosilane/phenyltrichlorosilane, and PA0 diphenyldichlorosilane/phenyltrichlorosilane.
As an example of the difficulty of separation, the boiling points of dimethyldichlorosilane and methyltrichlorosilane are approximately 4.degree. C. apart. Reducing the methyltrichlorosilane content of dimethyldichlorosilane to levels sufficient for the quality requirements of the silicone industry presently requires extensive distillation. Present distillation requires large capital outlays for distillation equipment and high energy consumption in the form of steam to generate the large reflux requirements needed to accomplish this difficult separation. To illustrate this point, Lucas in U.S. Pat. No. 2,594,946, issued Apr. 29, 1952, discloses that when one fractionally distills the dimethyldichlorosilane obtained from the direct process reaction of silicon with methyl chloride, even the most careful fractionation through a 200-plate column gives dimethyldichlorosilane which contains approximately 99.6 mole percent dimethyldichlorosilane and 0.4 mole percent methyltrichlorosilane.
Sauer, U.S. Pat. No. 2,421,653, issued June 3, 1947 discloses that halogensilanes and organopolysiloxanes undergo equilibration to form mixtures comprising some unreacted halogensilanes and organosiloxanes plus a mixture of polyorganohalogenosiloxanes. Brown and Hyde, U.S. Pat. No. 3,065,252, issued Nov. 20, 1962, discloses the interaction of halosilicon compounds with organosiloxanes in the presence of certain catalysts. The catalysts disclosed by Brown and Hyde are aminoalkyl-substituted organosilicon compounds and their salts with hydrogen halides, salts of monocarboxylic acids, aliphatic amines and hydrogen halide salts, and quaternary ammonium halides. Brown and Hyde, U.S. Pat. No. 3,101,361, issued Aug. 20, 1963, discloses the use of inert polar solvent such as halogenated hydrocarbons, nitriles, nitrohydrocarbons, and amides in the interaction of halosilanes and organopolysiloxanes. Wegehaput et al., U.S. Pat. No. 3,549,680, issued Dec. 22, 1970, discloses the use of certain phophorous-nitrogen compounds as catalysts in the reaction of halosilane compounds with organosiloxanes free of halogen to produce organohalosilane compounds differing from the starting materials. Bennett, U.S. Pat. No. 3,642,851, issued Feb. 15, 1972; and Bennett, U.S. Pat. No. 3,686,253, issued Aug. 22, 1972, disclose the redistribution of halosilanes with cyclotrisiloxane or cyclotetrasiloxane in the presence of a phosphine oxide or amine oxide catalyst. Bakassian et al., U.S. Pat. No. 3,646,088, issued Feb. 9, 1972, discloses the distribution between a siloxane and a chlorosilane carried out in hexaalkylphosphotriamide. Moretto et al., U.S. Pat. No. 4,073,801, issued Feb. 14, 1978, discloses a process in which a chlorosilane or partial hydrolysis product is reacted with a siloxane in the presence of an equilibration catalyst and hydrogen chloride to produce a stable polymeric product of comparatively high molecular weight or high chlorine content. Nowhere in the above references is there a suggestion or demonstration of the conversion of a more highly functional silanes to halopolyorganosiloxanes as part of a process to isolate and recover an organosilane of enhanced quality.
Volker et al., U.S. Pat. No. 4,113,760, issued Sept. 12, 1978, discloses an improved process for converting organosiloxane polymers to organohalosilanes which comprises reacting halosilanes with organopolysiloxanes in the presence of activated charcoal, and if desired an inorganic acid. Volker et al., also discloses that the process can also be used as a method for purifying silanes, especially silanes containing SiH which are contaminated with organic compounds. The silanes are hydrolyzed, distilled, and once more converted into silanes. Nowhere does Volker et al., disclose or suggest the conversion of the more highly functional impurity to a polyorganosiloxane while maintaining, isolating, and recovering a organosilane, unchanged.
Lucas, U.S. Pat. No. 2,594,946, issued Apr. 29, 1952, describes the recovery of essentially pure diorganodihalosilane from a mixture of diorganodihalosilane with organotrihalosilane. Recovery is effected by treating of said mixture with a diorganodiacyloxysilane.
Hyde, U.S. Pat. No. 2,738,359, issued Mar. 13, 1956, describes a chemical method for separating silicon tetrachloride and monoorganotrichlorosilanes from dichloro and monochloroorganosilanes. Separation is effected by the selective formation of crystalline non-volatile complexes of certain amides with silicon tetrachloride and alkyltrichlorosilanes. These crystalline complexes are insoluble in diorgano and triorganochlorosilanes. Similarly, British Pat. No. 1,096,476, published Dec. 29, 1967, describes a process for the separation of an organotrichlorosilane from a liquid mixture by contacting said mixture with tris(dimethylamino)phosphine oxide and separating the solid complex formed between the organotrichlorosilane and the phosphine.
Cahoy in German Pat. No. DE 1,087,601, issued Feb. 16, 1961, describes a process in which silicon tetrachloride and methyltrichlorosilane are selectively hydrolyzed from a mixture including trimethylchlorosilane as a major portion. Selective hydrolysis is effected by using water-dioxane mixtures. It has also been found that other water-miscible solvents such as polyethylene glycols and cresol are effective in the selective hydrolysis of trifunctional chlorosilane impurities from diphenyldichlorosilanes.
Japanese Patent Publication, No. 61-254594, published Nov. 12, 1986, discloses the purification of a mixture of organosilicon compounds by adding aprotic polar solvents and alkylene oxide compounds to react selectively with the most highly chlorinated organosilicon compound and distilling the resulting mixture to recover the less chlorinated organosilcon compound.
Tolentino, U.S. Pat. No. 4,421,926, issued Dec. 20, 1983, discloses the co-alkoxylation of halosilanes to facilitate separation of close-boiling halosilane materials, difficult to separate by conventional distillation, to facilitate separation of the corresponding alkoxysilanes, having greater boiling point differences. Tolentino discloses that all halosilanes will be converted to alkoxysilane.